1. Field of the Invention
The invention relates to apparatus for a video navigation plotter, specifically one designed for marine use, and to various methods for processing navigational data therein and displaying resulting navigational data thereon.
2. Description of the Prior Art
A pilot (mariner) of a marine vessel often requires navigational information. In the past and continuing to the present, the pilot would frequently mark suitable linked courses, that resembled linked line segments, on a marine navigational chart and proceed from an origination point and continue from one course to the next, until he reached his destination. Navigating in this manner necessitated that the pilot remain aware of his current position at all times so that he could determine when he reached the end of a course, i.e. a waypoint, and consequently needed to change his heading to match that required by a new course. Many pilots, particularly those in the past and to a far lesser extent now, obtained rudimentary positional information by a sextant sighting or from marine navigational charts by relying on making a comparison between a present nautical position and an indicated point on a coastline or a navigational buoy shown on a chart or by extrapolating a current position from a last known position that was marked on the chart using information specifying a heading, speed and elapsed time of the vessel from that point. While these methods of navigation are often quite simple, they relied on the mariner to accurately determine his current position, either using a sextant or through calculations, and as such were quite prone to human error and hence frequently yielded erroneous or at least imprecise results, and often required time to undertake.
In an effort to increase the accuracy of marine navigation for all mariners, the art turned to the use of radio navigational aids in an effort to automate the determination of current position information thereby decreasing human error. One such navigational aid, particularly suited for over-water use, relies on a system of geographically dispersed Loran C transmitters. This system includes a separate chain of one master and two or more secondary (slave) Loran C transmitters, located in each of number of different sections of the world, that transmit pulsed signals on 100 kHz, with a pre-defined group repetition rate, as an identifier, and with a pre-determined highly accurate temporal spacing occurring between each pulse transmitted from a master and a pulse transmitted from one of its secondary transmitters. A different predetermined spacing exists between the pulses respectively transmitted by the master and each of its secondary transmitters. Loran C relies on the basic assumption that radio waves propagate at a constant velocity through air. Hence, by measuring the slight difference between the times of arrival of a pulse emanating from the master station and one emanating from a selected slave station, the position of the Loran C receiver can be narrowed to anY position along a line of constant time difference (a T-D line). As such, a Loran C receiver measures the time differences occurring between a specified master and a selected secondary transmitter. Now, a position fix can be accurately determined by measuring the time differences between two pairs of Loran C transmitters, i.e. between a master and each of two of its secondary transmitters and finding the point of intersection of two corresponding T-D lines. A relatively unsophisticated Loran C receiver merely indicates a measured time difference. Use of Loran C eliminates the need to rely on potentially erroneous sextant sightings or course calculations given speed, bearing and elapsed time. As such, the use of Loran C significantly increases the accuracy inherent in marine navigation. Unfortunately, use of such Loran C receivers still involves a human element: a navigator has to set the receiver to receive a desired secondary Loran C transmitter, has to transfer the displayed time difference information onto a suitable nautical navigation chart that has printed T-D lines oftentimes interpolating between two adjacent printed T-D lines, and then has to determine the point of intersection of two T-D lines. To eliminate some of the tedium associated with Loran C navigation, sophisticated Loran C receivers exist that translate the time differences occurring between two pairs of Loran C transmitters into suitable latitude and longitude coordinates. In any event, once a position fix is determined, it still needs to be combined with drawn course lines so that the mariner could chart his progress and determine when he arrived at a waypoint so as to appropriately change his course heading. With either type of Loran C receiver, the presence of this human element still remains as a source of significant error. Moreover, this entire process of obtaining a position fix has to be frequently repeated at regular intervals so that the mariner is always aware of his current position. The tedious and repetitive nature of this endeavor consumes time; while the sheer repetition of the process proves to be another source of errors.
Therefore, while the art recognized that Loran C navigation could provide sufficiently accurate positional information when used correctly, the art also realized that accurate marine navigation can only result if human error inherent in a marine navigational process could be reduced as much as possible. As such, the art turned to various techniques aimed at automating as much of the entire navigational process as practical.
One such technique that is finding increasing use in the art is a video plotter. Early forms of these plotters provide visual display in terms of latitude and longitude lines of a limited geographic area of interest and indicate the current position of the mariner's vessel in that area. That position is frequently supplied either, in relatively simple systems, through direct manual entry of Loran C position information or automatically, as in sophisticated systems, through a direct digital connection to a suitable Loran C receiver. These systems also provide the capability of displaying courses and waypoints and changing the courses and waypoints at will. As such, these systems advantageously eliminate the need for manual transfer and interpolation of time difference information and course plotting and hence substantially reduce the sources of human error that could occur in marine navigation. Recently, video plotters are becoming available that also possess the ability to store a nautical coastline chart and other relevant navigational information, e.g. obstruction(s) electronic form and draw a cartographic display of coastline data of an area of interest which, in turn, is then overlaid with courses, waypoints and the current position of the vessel. This specific type of plotter will be referred to as video plotters with electronic charting.
While a video plotter, particularly with electronic charting, represents a significant step forward in reducing errors associated with marine navigation and simplifying the overall navigational process, such video plotters known in the art suffer various drawbacks that limit their utility.
First, those video plotters known in the art that rely on direct entry of navigational information, from for example a navigational receiver, are frequently cumbersome to use. Moreover, human error can occur in transposing bearing information from a Loran receiver into the video plotter and thereby corrupt the displayed information. Although direct entry video plotters tend to be less expensive than automatic entry video plotters, direct entry video plotters are often disfavored in most marine navigation applications unless cost is a primary consideration.
Second, video plotters known in the art tend to be rather inflexible. Specifically, video plotters with electronic charting rely on storing pre-defined numeric data that defines a two dimensional location of each separate point in a series of points, which when displayed, collectively forms all coastlines and relevant marine obstruction(s) that occur within a pre-defined geographic area. This data is obtained from a digitized nautical chart. Oftentimes, the digitized chart that is stored within such a video plotter and subsequently displayed covers too large an area, such as the coastline that includes the New York-Boston metropolitan area when in fact only a small area such as the New York harbor area is needed. To handle this situation, currently available video plotters with electronic charting frequently provide a user with the capability to "zoom" into a chart, i.e. change the scale factor of a selected portion of that chart, and thereby fill the entire display screen with a scaled up portion of the stored chart.
Unfortunately, the stored location data, depending upon the method in which it is stored within the plotter, may not be in a form that is amenable to easy scaling. As such, a significant amount of processing time, far more than would be tolerable to a user of the plotter, may need to be expending in changing the displayed scale of a portion of a chart so as to provide a "zoomed" display.
A video plotter with electronic charting that possesses this drawback is described in U.S. Pat. No. 4,428,057 (issued to J. Setliff et al on Jan. 24, 1984). This patent stores coastline information in a run length encoded form which encodes the number of adjacent locations in the stored chart as being adjacent locations of either land or water. While storage of data in this form does minimize memory size, an excessive amount of processing time is often required to manipulate and change the scaling of this encoded data.
To circumvent this processing time limitation, various video plotters with electronic charting provide fixed scaling. Specifically, these plotters would often store several pre-defined portions of a digitized chart in memory, wherein each stored portion has a different pre-set scale factor. As such, an operator would select the particular chart portion and the desired scale factor, if it is available for that chart portion. The plotter would merely access that scaled portion from memory and display it in cartographic form. Generally, to display one chart of coastline data with an adequately high resolution, a substantial number of stored locations is often necessary. Now, if fixed scaling is allowed, then to limit the size of the memory, only a very small number of chart portions each with a correspondingly small number of different scale factors is separately stored in the memory. While this technique markedly reduces processing time needed to generate the desired display, use of a small number of stored charts and chart portions disadvantageously constrains the mariner to be in certain geographic locations on a stored chart before he can zoom into any particular chart portion. Of course, if a mariner frequently travels through a certain area and needs a different chart portion or a portion with a different scale factor than is presently stored in the plotter, one solution would be for the mariner to obtain memory circuits that store the desired portion at the desired scaling. While this solution is likely to be adequate for those mariners who confine their travels to a small area, the need to change memory circuits renders this solution too cumbersome for any mariner that travels over a fairly broad area.
Use of fixed scaling and its inherent inflexibility are evident in the navigation system disclosed in U.S. Pat. No. 4,590,569 (issued to M. Rogoff on May 20, 1986). This system relies on partitioning a large chart into segments of a given pre-defined area, such as one square mile. If a larger area is desired for display, then all the segments that fill this area are accessed and displayed. Unless the chart is also broken into smaller segments, no segments smaller than the pre-defined area can be displayed. Unfortunately, video plotters with electronic charting, such as that described in the '569 Rogoff patent, that rely on use of a fixed scale factors, are often too inflexible and thus disfavored for use in many marine applications.
In addition, automatic entry video plotters known in the art are generally bulky and expensive.
Therefore, a need exists in the art for a compact and relatively inexpensive flexible automatic entry video plotter with electronic charting and specifically such a plotter that provides variable scaling. Furthermore, a need exists in the art for methods of storing and manipulating digitized cartographic data, for example coastlines and nautical obstructions, for use within in such a video plotter that easily permits that data to be displayed at a variable scale factor without requiring excessive processing time. Such a method should also advantageously compress the stored data such that several different charts, at various resolutions, can be stored within the plotter.